The invention relates to rotary drill bits for use in drilling or coring holes in subsurface formations, and particularly to polycrystalline diamond compact (PDC) drag bits.
A rotary drill bit of the kind to which the present invention relates comprises a bit body having a shank for connection to a drill string and a passsage for supplying drilling fluid to the face of the bit, which carries a plurality of preform cutting elements each formed, at least in part, from polycrystalline diamond. One common form of cutting element comprises a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the front cutting face of the element, bonded to a substrate which is usually of cemented tungsten carbide.
The bit body may be machined from solid metal, usually steel, or may be moulded using a powder metallurgy process in which tungsten carbide powder is infiltrated with metal alloy binder in a furnace so as to form a hard matrix.
While such PDC bits have been very successful in drilling relatively soft formations, they have been less successful in drilling harder formations and soft formations which include harder occlusions or stringers. Although good rates of penetration are possible in harder formations, the PDC cutters suffer accelerated wear and bit life can be too short to be commercially acceptable.
Recent studies have suggested that the rapid wear of PDC bits in harder formations is due to chipping of the cutters as a result of impact loads caused by vibration, and that the most harmful vibrations can be attributed to a phenomenon called "bit whirl". ("Bit Whirl--A New Theory of PDC Bit Failure"--paper No. SPE 15971 by J. F. Brett, T. M. Warren and S. M. Behr, Society of Petroleum Engineers, 64th Annual Technical Conference, San Antonio, Oct. 8-11, 1989). Bit whirl arises when the instantaneous axis of rotation of the bit precesses around the central axis of the hole when the diameter of the hole becomes slightly larger than the diameter of the bit. When a bit begins to whirl some cutters can be moving sideways or backwards relatively to the formation and may be moving at much greater velocity than if the bit were rotating truly. Once bit whirl has been initiated, it is difficult to stop since the forces resulting from the bit whirl, such as centrifugal forces, tend to reinforce the effect.
Attempts to inhibit the initiation of bit whirl by constraining the bit to rotate truly, i.e., with the axis of rotation of the bit coincident with the central axis of the hole, have not been particularly successful.
Although it is normally considered desirable for PDC drill bits to be rotationally balanced, in practice some imbalance is tolerated. Accordingly it is fairly common for PDC drill bits to be inherently imbalanced, i.e. when the bit is being run there is, due to the cutting, hydraulic and centrifugal forces acting on the bit, a resultant force acting on the bit, the lateral component of which force, during drilling, is balanced by an equal and opposite reaction from the sides of the borehole.
This resultant lateral force is commonly referred to as the bit imbalance force and is usually represented as a percentage of the weight-on-bit since it is almost directly proportional to weight-on-bit. It has been found that certain imbalanced bits are less susceptible to bit whirl than other, more balanced bits. ("Development of a Whirl Resistant Bit"--paper No. SPE 19572 by T. M. Warren, Society of Petroleum Engineers, 64th Annual Technical Conference, San Antonio, Oct. 8-1989). Investigation of this phenomenon has suggested that in such less susceptible bits the resultant lateral imbalance force is directed towards a portion of the bit gauge which happens to be free of cutters and which is therefore making lower "frictional" contact with the formation than other parts of the gauge of the bit on which face gauge cutters are mounted. It is believed that, since a comparatively low friction part of the bit is being urged against the formation by the imbalance force, slipping occurs between this part of the bit and the formation and the rotating bit therefore has less tendency to precess, or "walk", around the hole, thus initiating bit whirl.
(Although, for convenience, reference is made herein to "frictional" contact between the bit gauge and formation, this expression is not intended to be limited only to rubbing contact, but should be understood to include any form of engagement between the bit gauge and formation which applies a restraining force to rotation of the bit. Thus, it is intended to include, for example, engagement of the formation by any cutters or abrasion elements which may be mounted on the part of the gauge being referred to.)
This has led to the suggestion, in the above-mentioned paper by Warren, that bit whirl might be reduced by omitting cutters from one sector of the bit face, so as deliberately to imbalance the bit, and providing a low friction pad on the bit body for engaging the surface of the formation in the region towards which the resultant lateral force due to the imbalance is directed.
Experimental results have indicated that this approach may be advantageous in reducing or eliminating bit whirl. However, the omission of cutters from one sector of a PDC bit can have disadvantages. Not only does it reduce the maximum number of cutters which can be mounted on the bit but it also imposes serious limitations on the disposition of cutters and on bit design in general. In other words, other desirable characteristics of the PDC bit may have to be sacrificed in order to permit the omission of cutters from one sector of the bit face.
The present invention therefore sets out to provide various mehtods whereby the desirable imbalance of a PDC bit may be achieved while still allowing substantial freedom in the disposition of cutters on the face of the bit and, in some cases, no reduction in the maximum number of cutters which may be employed on the bit. cl SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided a rotary drill bit comprising a bit body having a shank for connection to a drill string and a passage for supplying drilling fluid to the face of the bit, which carries a plurality of preform cutting elements each formed, at least in part, from polycrystalline diamond, some cutting elements on the bit body having their cutting faces at a different angular orientation from the cutting faces of other cutting elements on the bit body, with respect to the axis of rotation of the bit, the different angular orientations of the cutting faces of the respective cutting elements being so selected that the vectorial sum of the reaction forces between the formation being drilled and the cutting elements provides a resultant lateral imbalance force acting on the bit body as it rotates in use, the gauge of the bit body including at least one low friction bearing pad so located as to transmit said resultant lateral force to the part of the formation with which the bearing pad is for the time being engaged.
The front cutting face of a PDC cutting element is disposed at an angle to the surface of the formation being cut, as viewed in a plane normal to the formation and extending in the direction of movement of the cutter relative to the formation. In the case where the cutting face leans forwardly in the direction of movement with respect to the formation, the angle which the cutting face makes to the normal is referred to as a negative back rake angle. Different back rake angles produce different forces acting on the bit as a result of the interaction between the cutter and the formation.
Accordingly, in one embodiment of the invention some cutting elements have their cutting faces orientated at a different back rake angle from the back rake angle of other cutting elements on the bit body, the different back rake angles of the respective cutting elements being so selected as to provide said resultant lateral imbalance force.
It is also well known to provide PDC cutting elements with side rake. If the cutting element is orientated so as to tend to urge cuttings outwardly towards the periphery of the drill bit, this may be referred to as positive side rake, whereas if the cutting element is orientated to tend to urge cuttings inwardly towards the axis of the drill bit this may be referred to as negative side rake. Both negative and positive side rake tend to apply a lateral force to the bit body in use, and PDC bits are normally designed so that the forces due to side rake cancel out so that there is no resultant lateral force acting on the bit.
According to an embodiment of the present invention some cutting elements may have their cutting faces orientated at a different side rake angle from the side rake angle of other cutting elements on the bit body, the different side rake angles of the respective cutting elements being so selected as to provide said resultant lateral imbalance force.
For example, cutting elements on one side of a diameter of the bit may have a different side rake angle from cutting elements on the other side of the diameter. In this case, cutting elements on said one side of the diameter may have positive side rake and cutting elements on the other side of the diameter may have negative side rake, said low friction bearing pad being located on the same side of the diameter as those cutting elements having negative side rake.
According to another aspect of the invention there is provided a rotary drill bit comprising a bit body having a shank for connection to a drill string and a passage for supplying drilling fluid to the face of the bit, which carries a plurality of preform cutting elements each formed, at least in part, from polycrystalline diamond, the centre of gravity of the bit body being offset offset from the central axis of rotation of the bit body so as to apply a resultant lateral imbalance force to the bit body as it rotates in use, the gauge of the bit body including at least one low friction beaing pad so located as to transmit said resultant lateral force to the part of the formation which the bearing pad is for the time being engaging.
The centre of gravity of the bit body may be offset from the central axis of rotation by the inclusion in the bit body of a mass of material which is asymmetrically disposed with respect to the central axis of the bit.
For example, the bit body may comprise a solid infiltrated matrix moulded around a steel blank, the steel blank including a cavity asymmetrically disposed with respect to the axis of rotation of the bit, the cavity being filled with denser material, for example matrix material, which thereby constitutes the aforesaid mass of material.
Alternatively, the bit body may comprise a solid infiltrated matrix moulded around a steel blank, a portion of the matrix asymmetrically offset from the axis of the bit being of different density from the rest of the matrix.
In another embodiment the bit body is machined from steel and is formed with a cavity which is asymmetrically offset from the axis of the bit, said cavity being filled with a body of a material of different density from the steel from which the rest of the bit body is formed.
It will be appreciated that certain of the different aspects of the present invention referred to above may be combined to produce the required effect .